Field of the Invention
Apparatuses and methods consistent with the present disclosure relate to an analog to digital converter including a differential VCO, and more particularly, to an analog to digital converter including a differential VCO that uses a phase domain and reduces an area and power consumption.
Description of the Related Art
Recently, there is an increasing demand for multi-mode or multi-band in wireless communication. To this end, researches on a digital RF front-end are actively under way. In particular, to support the multi-mode or the multi-band, we are trying to reduce a use of a fixed filter such as a SAW filter as much as possible and to use a flexible type filter or to embed a filter function in a device.
To implement the digital RF front-end, an RF signal itself should be sampled and digitized, and the so digitized signal has greatly improved flexibility and reconfigurability. Therefore, many researchers are studying a data conversion device capable of performing high-speed sampling. In particular, a delta sigma modulator is being actively researched.
FIG. 1A is a diagram illustrating a schematic configuration of the existing frequency based VCO delta sigma modulator.
Referring to FIG. 1A, the existing frequency based VCO delta sigma modulator, that is, a frequency type delta sigma modulator receives voltage information of a VCO and samples output frequency information of the VCO. Further, the VCO is used as a function of the quantizer, and therefore replaces a general comparator.
However, since a V-to-frequency transfer function of the VCO is operated non-linearly, there is a disadvantage in that a harmonic distortion occurs in an output signal.
To solve the problem, a phase frequency based VCO delta sigma modulator that compares a phase between a reference signal and a VCO output signal has been proposed.
FIG. 1B is a diagram illustrating a schematic configuration of the existing phase frequency based VCO delta sigma modulator.
Referring to FIG. 1B, the existing phase frequency based VCO delta sigma modulator, that is, the phase frequency type delta sigma modulator receives the voltage information of the VCO and samples output phase information of the VCO.
The phase frequency type delta sigma modulator has an advantage in that a function of a comparator and an integrator may be implemented simultaneously because a V-to-phase transfer function has the integration relation. In addition, the phase frequency type delta sigma modulator has an advantage in that nonlinear characteristics of the VCO input/output is less affected than that of the existing frequency type delta sigma modulator and thus the harmonic distortion appearing in the output signal is reduced.
Meanwhile, the VCO generally consists of digital logic. Characteristics of the digital logic may be changed very greatly depending on process or temperature conditions. If the conditions are changed, a frequency appearing at the output of the VCO appears different even when the same DC voltage is applied to the input of the VCO. At this point, when a reference frequency and a common frequency of the VCO are different from each other, a DC offset occurs in the output signal of the converter. The DC offset limits an output range of a feedback digital to analog converter and may lead to a reduction in a dynamic range, which limits the performance of the analog to digital converter.